Why MaterialBillboard components cannot write to stencil buffer?

Hi,

I try to set a value ins the custom stencil, although it works fmy other objects, if I use a MaterialBillboard, nothing appears in the stencil buffer

I use a material like this
image

image

image

and I setup my billboard in an actor with these settings

image

any help appreciated

[edit]

as a workaround, I use a regular plane, a material function that billboards it and voila

yet the question remains, why is it not working with built in MaterialBillboard components ?